A conventional phase-change recording medium that is repeatedly rewritable and an optical recording method for such a medium will be explained below.
For recording, a laser beam pulse is applied (emitted) to a phase-change recording medium to melt and quench a recording layer and form an amorphous recorded mark on the recording layer. The reflectance of this recorded mark is lower than that of a crystallized state of the recording layer and is optically readable as recorded information. To erase the recorded mark, laser power smaller than recording power is applied to heat the recording layer to a temperature above a crystallization temperature and below a melting point, or a temperature above the melting point. Thereafter, the recording layer is annealed to change the amorphous state into a crystallized state. This realizes overwriting.
Materials favored for such a recording layer include a Ge—Sb—Te alloy and an Ag—In—Sb—Te alloy due to their quick crystallization speed. These recording layer materials are used for CD-RWs, DVD-RAMs, DVD-RWs, DVD+RWs, and the like. (Hereinafter, CD indicates a compact disk and DVD a digital versatile disk.) An optical recording medium employing a recording layer made of such recording materials usually has a dielectric layer, which is heat resistive and transparent, on each face of the recording layer, to prevent the deformation and cracking of the recording layer during recording.
If the dielectric layer contains a ZnS component, an interface layer made of nitride or oxide may be arranged on one or both faces of the recording layer, to prevent S from penetrating the recording layer. On the dielectric layer opposite to a laser beam incident direction, a metal reflective layer mainly containing Al, Ag, or the like is laminated, to realize a high reflectance.
Optical recording methods of recording information on the phase-change recording medium of the above-mentioned configuration are disclosed in Japanese Patent Publication No. 2765938, Japanese Patent Publication No. 3266971, and Japanese Patent Publication No. 3171103 explained below. These recording methods modulate laser beam pulses of high power according to mark lengths to be recorded on a recording layer of a phase-change recording medium. To prevent mark thickening at a mark trailing edge of a long mark, these methods employ a multi-pulse optical recording strategy that divides a pulse within a clock period T.
FIG. 1 is a view showing recording pulses according to a multi-pulse strategy of a conventional optical recording method. In FIG. 1, T represents a clock period.
As shown in FIG. 1, the laser power of a laser beam is modulated with three values, i.e., recording power Pw, erase power Pe, and bias power Pb (Pw>Pe>Pb), to generate recording pulses corresponding to recording signals (rectangular pulses) having lengths 3T and 5T, thereby forming marks having lengths 3T and 5T on a recording layer of a medium.
The recording pulses corresponding to the rectangular pulse of length 3T are multi-pulses consisting of a top pulse Ttop, an intermediate pulse Tmp, and a rear-end pulse Tcl arranged in series. The recording pulses corresponding to the rectangular pulse of length 5T are multi-pulses consisting of a top pulse Ttop, three intermediate pulses Tmp, and a rear-end pulse Tcl arranged in series.
Recent phase-change recording media have high recording density, and therefore, need high recording speed. In practice, rewritable DVD media (DVD-RAMs, DVD-RWs, DVD+RWs, and the like) involve a maximum linear speed of 2.4 times (in terms of DVD speed) a normal recording speed (=single speed=3.5 m/s). Even at such a high multiplied speed, completely writing a 4.7-GB DVD medium needs 25 minutes, which is not sufficiently practical.
To improve practicality, the recording speed must further be increased. Even if the recording speed is increased, factors mentioned below prevent marks from being properly recorded on a recording layer of a DVD medium.
A first factor is that increasing a recording linear speed necessitates increasing write laser power (laser strength). Consumer lasers of relatively low price used for DVD medium recorders and the like hardly provide laser strength sufficient for high-speed recording.
A second factor is that the responsibility of divided pulses (multi-pulses) of recording laser power is poor. For example, a sextuple linear speed (21 m/s) for a DVD medium corresponds to a clock period of about 6.3 ns. With this clock period, the recording strategy (FIG. 1) using conventional multi-pulses needs a short pulse width of about 2.5 ns calculated from pulse width=clock period (sextuple speed of 6.3 ns)×duty factor (for example, 40%).
A current laser needs about 3.0 ns in total for the rise and fall of a rectangular pulse. Accordingly, a pulse having the above-mentioned pulse width of about 2.5 ns becomes an incomplete rectangular wave, or a triangular wave whose peak power is below a set value (recording power level Pw or bias power level Pb). Then, it is difficult to apply a set heat quantity to the recording layer of a medium. This results in forming a mark not having a specified shape.
A third factor is that the material of a recording layer of a DVD medium is incapable of coping with high-speed divided pulses of recording laser power. To cope with high-speed recording, the material of a recording layer tends to be adjusted in a direction of easy crystallization. This is equal to a direction of making it difficult to achieve an amorphous state at a given cooling speed.
When high-speed divided pulses are used, a time for applying divided pulses extends as the length of a mark elongates, to achieve an annealing thermal history as a whole. Accordingly, a recording layer made for quick crystallization speed easily crystallizes. Then, the longer the length of a mark, the thinner the mark becomes, to provide an insufficient signal amplitude.
To solve the problem that improving a recording speed results in deteriorating the shape of a mark recorded on a recording layer, there is an optical recording method that employs a string of 2T-based recording pulses. Optical Data Storage Conference 2000 (ODS '00), Technical Digest PD1, “High speed rewritable DVD up to 20 m/s with nucleation-free eutectic phase-change material of Ge (Sb70Te30)+Sb” discloses an optical recording method that conducts 4.8×-speed recording on a DVD medium by providing high-speed divided pulses of write laser power such that a recording power period reaching a recording power level Pw and a bias power period reaching a bias power level Pb are contained within a double clock period of 2T.
FIG. 2 is a view showing a 2T-based recording pulse string according to the Technical Digest PD1 of ODS '00.
As shown in FIG. 2, marks having lengths 4T and 8T corresponding to even numbers (4 and 8) of times the clock period T, respectively, are formed by applying N (=n/2=2 and 4 (n corresponding to a mark length)) rectangular pulses serving as recording pulses to the laser power of a recording laser beam. To form marks having lengths 3T and 9T corresponding to odd numbers (3 and 9) of times the clock period T, N (=(n−1)/2=1 and 4 (n corresponding to a mark length)) rectangular pulses serving as recording pulses are applied to the laser power of a recording laser beam.
The optical recording method employing 2T-based recording pulse strings expands the width of each divided recording pulse. Compared with the conventional method, this method can secure the rise and fall time of a recording pulse of recording laser power, to provide a complete rectangular pulse. As a result, the rectangular pulse can have a peak level that sufficiently reaches the recording power level Pw or the bias power level Pb. Extending a period of bias power results in achieving a quenching schedule to provide a specified recording pulse signal amplitude with a consumer laser having relatively low recording power.
When forming a mark of length 4T corresponding to a length four times the clock period T, two pulses must be included within a recording signal period of 4T. At this time, it is difficult to sufficiently extend a period of bias power. Namely, it is difficult to secure a sufficient annealing time. This may cause no problem for 4.8×-speed recording on a DVD medium. However, for 6×-speed recording on a DVD medium, the clock period T is further shortened to make it difficult to secure sufficient recording conditions for the above-mentioned optical recording method.
A PULSTEC MSG2B signal generator already marketed incorporates a circuit for carrying out a multi-pulse strategy based on 2T-based recording pulse strings with pulse-width-changed recording pulses generated within a double clock period of 2T.
FIG. 3 is a view showing 2T-based recording pulse strings incorporated in the PULSTEC MSG2B signal generator.
Like the optical recording method (FIG. 2) employing divided pulses of recording power and bias power contained within 2T, the optical recording method (FIG. 3) can secure a specified recording pulse signal amplitude with relatively low recording power.
However, the end of a recording pulse corresponding to a mark length includes a modulated pulse (a relatively thin pulse in FIG. 3) within a clock period of 1T. This results in an insufficient bias power time and an insufficient annealing time corresponding thereto.
This may cause no annealing problem for 4.8×-speed recording on a DVD medium. However, for 6×-speed recording on a DVD medium, the clock period T is further shortened to cause the annealing problem and insufficient recording characteristics.
When recording information on a DVD medium at or above 6× linear speed, the multi-pulse strategy employing 2T-based recording pulse strings causes the problem of insufficiently keeping a bias power time at the end of a recording pulse. This causes a shortage of annealing time and a thinning phenomenon at the end of a mark recorded on a recording layer. This is the problem to be solved by the present invention.
If there is an optical recording method capable of solving the problem of an insufficient bias power time and securing a sufficient bias power time as well as a sufficient annealing time, the thinning phenomenon at the end of a mark recorded on a recording layer of a DVD medium will not occur.
One solution may be to particularly consider the rise of a top pulse or the fall of a rear-end pulse in the multi-pulse strategy employing 2T-based recording pulse strings. This solution, however, involves an inconvenience of causing recording characteristic differences between recording pulses for forming a mark (having a length of, for example, 2T, 4T, and the like) corresponding to an even number of recording clock periods T and recording pulses for forming a mark (having a length of, for example, 3T, 5T, and the like) corresponding to an odd number of recording clock periods T.
More precisely, like the optical recording method shown in FIG. 2, multi-pulses are formed so that a time for recording power Pw and a time for bias power Pb are contained within 2T. If the trailing edge of a recording pulse is adjusted to the training edge of a recording signal, the width of an intermediate pulse existing between a top pulse and a rear-end pulse must be adjusted. For this, an adjusting width of (½)T is needed.
The optical recording method of FIG. 3 adjusts the rise of a top pulse of recording pulses to a position 1T after the rise of a recording signal. Accordingly, if the trailing edge of a recording pulse is adjusted to the trailing edge of the recording signal, the width of an intermediate pulse existing between the top pulse and rear-end pulse must be adjusted. For this, an adjusting width of (½)T is needed.
According to a PWM method, the width of a recorded mark has information. Accordingly, it is necessary to record a mark without distortion. Namely, it is necessary to record a mark on a recording film in a front-rear symmetrical manner. A part of a disk irradiated with a laser beam when recording a signal involves a higher temperature at an end point than a start point due to a heat accumulating effect. As a result, a recorded mark is wider at its rear end than at its front end. Namely, the recorded mark has a teardrop shape having a thin front end and a thick rear end. This sort of thermal recording is known to change recording conditions depending on recording speed variations.
In connection with this, Japanese Unexamined Patent Application Publication No. 2001-209940 of this applicant records temporal information (strategy) with pre-pits on a disk, the temporal information including maker's optimum recording power, optimum erase power, a top pulse width, an intermediate multi-pulse width, a rear-end pulse width, and the like. This is because each disk has specific optimum recording power, optimum erase power, a top pulse width, an intermediate multi-pulse width, a rear-end pulse width, and the like. Due to a rapid advance of multiplied speed, there is available information for 1× speed (a linear speed of 3.49 m/s) but no information is available for high multiplied speed. For example, if there is a disk compatible with three multiplied speeds of 1×, 2× (a linear speed of 3.49*2 m/s), and 4× (a linear speed of 3.49*4 m/s) and if there is information only for 1× speed, an apparatus compatible with 1×, 2×, and 4× speeds can record information on the disk only at 1× speed, or at 4× speed through time consuming processes of testing recording power and strategy and finding optimum power for 4× speed. However, keeping a user waiting several tens of seconds is not acceptable for a commercial commodity.
If there is a disk compatible with 1×, 2×, and 4× speeds and if there is only information about 1× and 4× speeds, an apparatus compatible with 1×- and 2×-speed recording can record information on the disk only at 1× speed, or at 2× speed through time consuming processes of testing recording power and strategy and finding optimum power for 2× speed.
For a high-density recordable medium in the future, it is necessary to record recording conditions, in particular, strategies for various rotation speeds of the disk. Then, it will be possible to start recording at any speed and shorten a start time. This is an essential requirement for replacing VTRs with disks.
To record such recording conditions on a disk, there is an areal limit on the disk. Expecting, for example, 24×-speed recording, current CD-Rs may secure an area for storing recording conditions related to 1×, 2×, 4×, 6×, 8×, 12×, and 16× speeds. For DVD-R/RWs, however, it is unknown how many years it will take to reach such multiple speeds. It is also unknown the highest multiplied speed for which the disks must be prepared. There is a problem of wasting the area for high multiplied speeds until the speeds are materialized. Namely, there is a requirement to effectively use the area now and in the future.
The present invention has been devised in consideration of these requirements. An object of the present invention is to provide an optical recording method of conducting a recording strategy using multi-pulses on, for example, a DVD medium at a high linear speed equal to or higher than 6× speed. The method solves the problem of an insufficient bias power time that may result in a shortage of annealing time and an incomplete shape of a mark. To form a mark having a correct shape, the method does not adjust the position of a top pulse or a rear-end pulse of multi-pulses. Instead, the method always synchronizes the fall of a recording signal having a length nT with the fall a rear-end pulse of multi-pulses by controlling the width of a top pulse of the multi-pulses that rises in synchronization with the rise of the recording signal according to whether the length of the recording signal is odd or even. In addition, the method employs intermediate pulses whose number and position are determined according to the length of the recording signal. Also provided is an optical recording medium used for high linear speed recording achieved by the optical recording method and having recording characteristics capable of forming a mark of specified shape. Also provided is an optical recording apparatus used for conducting recording according to the optical recording method and having a determination unit to determine whether or not the length of a recording signal is even or odd. Also provided is an optical recording medium recording apparatus for forming a mark of specified shape on an optical recording medium according to the optical recording method.
Another object of the present invention is to provide an optical disk capable of coping with rapidly advancing high multiplied speed without changing standards, consuming environmental energy, or modifying equipment design, capable of realizing smooth standardization to expedite market advancement, and capable of coping with multiple speeds while eliminating useless information, effectively improving redundancy, and effectively using areas.
Still another object of the present invention is to provide an optical disk recording/reproducing apparatus capable of grasping multiple speeds with which an optical disk to record or reproduce copes, selecting an optimum multiplied speed for the optical disk to record information thereon, dropping an equipment's standard multiplied recording speed to a recordable speed if the optical disk is suffered from a temperature increase, surface misalignment, or eccentricity, and easily, quickly, reliably reading information from an optical disk that is compatible with multiple speeds, has no useless information, and includes high redundancy.
Still another object of the present invention is to provide an optical disk having a recording management information area and an information recording area. Recording power and recording strategy information for recording data in the information recording area modulate laser power with recording power Pw, erase power Pe, bias power Pb (Pw>Pe>Pb) to form a mark corresponding to data having a length nT (n being an integer and T being a clock period of recording data) according to a linear speed at which the data is recorded on the optical disk. There are a plurality of the recording management information areas corresponding to multiplied-speed information, to beforehand store recording strategy information for changing, at a low linear speed, the time widths of a top pulse, an intermediate multi-pulse, and a rear-end cooling power section among divided recording pulses for forming a mark, as well as recording strategy information for changing, at a high linear speed, the time widths of a bias power section, a rear-end recording pulse section, and a cooling power section among divided recording pulses for forming a mark according to a period pT or a period 2T, i.e., according to a clock based on the length of recording data whose basic period is pT obtained by dividing the frequency of a clock period T by p (or recording data of a period 2T obtained by halving the frequency of the clock period T). Also provided is an optical disk recording/reproducing apparatus for recording/reproducing data to and from the optical disk, reading recording power and recording strategy information corresponding to an objective multiplied speed from the recording power and recording strategy information pieces beforehand recorded in the information management areas of the optical disk, and according to the read recording power and record strategy, recording information in the information recording area of the optical disk.